DOCTOR OF PHILOSOPHY · 2020. 3. 9. · In order to improve robustness of AHR powder for screw...

DOCTOR OF PHILOSOPHY

Pyrolysis and gasification of biomass andacid hydrolysis residues

Manisha Patel

2013

Aston University

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PYROLYSIS AND GASIFICATION OF BIOMASS AND ACID

HYDROLYSIS RESIDUES

MANISHA PATEL

Doctor of Philosophy

Chemical Engineering

ASTON UNIVERSITY

June 2013

© Manisha Patel, 2013

Manisha Patel asserts her moral right to be identified as the author of this thesis

This copy of the thesis has been supplied on condition that anyone who consults it is

understood to recognise that its copyright rests with its author and that no quotation

from the thesis and no information derived from it may be published without proper

acknowledgement.

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Aston University

Pyrolysis and Gasification of Biomass and Acid hydrolysis Residues

Manisha Patel

Doctor of Philosophy

2013

THESIS SUMMARY

This research was carried for an EC supported project that aimed to produce ethyl levulinate as a diesel miscible biofuel from biomass by acid hydrolysis. The objective of this research was to explore thermal conversion technologies to recover further diesel miscible biofuels and/or other valuable products from the remaining solid acid hydrolysis residues (AHR).

AHR consists of mainly lignin and humins and contains up to 80% of the original energy in the biomass. Fast pyrolysis and pyrolytic gasification of this low volatile content AHR was unsuccessful. However, successful air gasification of AHR gave a low heating value gas for use in engines for power or heat with the aim of producing all the utility requirements in any commercial implementation of the ethyl levulinate production process.

In addition, successful fast pyrolysis of the original biomass gave organic liquid yields of up to 63.9 wt.% (dry feed basis) comparable to results achieved using a standard hardwood. The fast pyrolysis liquid can be used as a fuel or upgraded to biofuels.

A novel molybdenum carbide catalyst was tested in fast pyrolysis to explore the potential for upgrading. Although there was no deoxygenation, some bio-oil properties were improved including viscosity, pH and homogeneity through decreasing sugars and increasing furanics and phenolics.

AHR gasification was explored in a batch gasifier with a comparison with the original biomass. Refractory and low volatile content AHR gave relatively low gas yields (74.21 wt.%), low tar yields (5.27 wt.%) and high solid yields (20.52 wt.%). Air gasification gave gas heating values of around 5MJ/NM3, which is a typical value, but limitations of the equipment available restricted the extent of process and product analysis.

In order to improve robustness of AHR powder for screw feeding into gasifiers, a new densification technique was developed based on mixing powder with bio-oil and curing the mixture at 150°C to polymerise the bio-oil.

Keywords: Miscanthus, sugarcane bagasse, fast pyrolysis, catalytic pyrolysis, pelletisation

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To my parents and bhai

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ACKNOWLEDGEMENTS

Firstly, I would like to thank Prof. Tony Bridgwater who has been a pleasure to

work with and without him; this PhD would not have been possible. I am truly grateful

for his guidance, support and the things he has taught me over the last few years. I will

never forget all he has done for me.

The European Commission is acknowledged for financial support of the

research carried out under the FP7 DIBANET project “The Production of Sustainable

Diesel-Miscible-Biofuels from the Residues and Wastes of Europe and Latin America”;

(Grant number 227248-2). I would like to acknowledge the Dibanet partners for

providing interesting discussions at project meetings and creating fond memories in

wonderful places such as Argentina, Brazil, Chile, Greece and Ireland. CTC in Brazil

and University of Limerick and are acknowledged for supplying feedstocks for testing. I

would especially like to thank Dan Hayes for his advice and comments.

I would like to thank Prof. Victor Teixeira da Silva from UFRJ, in Brazil for the

collaborative catalytic pyrolysis part of this work. Victor was a joy to work with and

offered some valuable advice. Special thanks also to Richard Marsh at Cardiff

University for providing access to a batch gasification unit. I would also like to thank

Penny Challans and Angharad Beurle-Williams for support and making me feel

welcome during my visit to Cardiff.

I would like to express special thanks to past and present members of the

Bioenergy Research Group including Alejandro Alcala, Scott Banks, Ana Maria Cortes,

Daniel Nowakowski, Javier Celaya, Sarah Alexander, Emma Wylde and Irene

Watkinson for creating an enjoyable working environment. I am grateful to Surila

Darbar who provided assistance with analytical equipment and Panos Doss for helping

with the pelletisation work. Also, a word of thanks to Abba Kalgo, Antzela Fivga and

Allan Harms for their help in the laboratory during the early stages of my PhD.

I wish to express my gratitude to Chris Mykoo for countless hours of fun, for

being such a great listener and most of all, for making me smile when I felt down. I

would also like to thank my close friend Sarah Carnell for all those trips out for

‘Afternoon tea’. My other friends and extended family are thanked for several trips to

Nandos and cocktail bars when I needed a break.

I am deeply grateful to my parents and my big brother, to whom I have also

dedicated this thesis, for their endless support, love and encouragement. I would like to

sincerely thank my mom who made my favourite meals and my dad who was always

on standby in case I needed anything. I will always be grateful to my big brother, we

have always been close and I always knew I could count on him for anything.

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CONTENTS THESIS SUMMARY 2

ACKNOWLEDGEMENTS 4

LIST OF FIGURES 8

LIST OF TABLES 12

ABBREVIATIONS 14

1 INTRODUCTION 15

1.1 Dibanet project overview 15

1.2 Dibanet scientific research objectives 15

1.3 Scientific research objectives 17

1.4 Structure of thesis 18

2 BIOMASS AND ACID HYDROLYSIS RESIDUES TYPES 19

2.1 Biomass 19

2.1.1 Biomass components 19

2.1.2 Biomass tested 22

2.2 Acid hydrolysis of biomass 24

2.2.1 Acid hydrolysis residues components 25

2.2.2 Acid hydrolysis residues tested 26

2.3 Interim conclusions 29

3 BIOMASS AND ACID HYDROLYSIS RESIDUE ANALYSIS 30

3.1 Proximate and ultimate analysis 32

3.2 Composition and energy content of agreed AHR 44

3.3 Sample preparation for analysis and thermal processing 45


4 THERMAL CONVERSION PROCESSES SUMMARY 51

4.1 Combustion 51

4.2 Pyrolysis 51

4.3 Gasification 52

5 THEORY AND LITERATURE REVIEW: FAST PYROLYSIS AND BIO-OIL UPGRADING 55

5.1 Background 55

5.2 Fast pyrolysis process variables 61

5.3 Feedstock variables 66

5.4 Fast pyrolysis of biomass 67

5.5 Fast pyrolysis of AHR 69

5.6 Bio-oil upgrading 71


6 FAST PYROLYSIS OF BIOMASS 79

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6.1 Fast pyrolysis units 79

6.2 Fast pyrolysis methodology 80

6.2.1 100g/h 80

6.2.2 300g/h rig 83

6.2.3 1kg/h rig 89

6.2.4 Product analysis 91

6.3 Fast pyrolysis results and discussion 94

6.3.1 100g/h rig 100

6.3.2 300g/h rig 102

6.3.3 1kg/h rig 112


7 CATALYTIC PYROLYSIS OF BIOMASS 119

7.1 Methodology 119

7.1.1 Py-GC-MS 119

7.1.2 Catalytic fast pyrolysis on the 300g/h fluidised bed system 120

7.2 Results and discussion 121

7.2.1 Py-GC-MS 121

7.2.2 Catalytic fast pyrolysis on the 300g/h fluidised bed system 122


8 THEORY AND LITERATURE REVIEW: GASIFICATION AND GAS UPGRADING 127

8.1 Background 127

8.2 Gasification process variables 130

8.3 Feedstock variables 137

8.4 Gasification of biomass 138

8.5 Gasification of AHR 139

8.6 Gas upgrading 139


9 GASIFICATION OF BIOMASS AND ACID HYDROLYSIS RESIDUES 144

9.1 Gasification methodology 144

9.1.1 Continuous fluidised bed gasification 144

9.1.2 Batch gasification 145

9.2 Gasification results and discussion 149

9.2.1 Summary of gasification experiments 149

9.2.2 Continuous pyrolytic gasification 149

9.2.3 Comparison of batch and continuous pyrolytic gasification 154

9.2.4 Comparison of batch pyrolytic and air-blown gasification 155

9.2.5 Batch air-blown gasification 156

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9.3.1 Pyrolytic gasification 166

9.3.2 Batch air-blown gasification 167

10 IMPROVING FEEDING AND HANDLING PROPERTIES OF POWDERED ACID HYDROLYSIS RESIDUES 169

10.1 Background 169

10.1.1 Paste feeding 169

10.1.1 Pelletisation 169

10.2 Methodology 172

10.2.1 Paste feeding AHR 172

10.2.2 Pelletisation of AHR and SCB 172

10.3 Results and discussion 174

10.3.1 Paste feeding AHR 174

10.3.2 Pelletisation of AHR 177


11 CONCLUSIONS 187

12 RECOMMENDATIONS 194

12.1 Fast pyrolysis of biomass 194

12.2 Gasification of biomass and AHR 194

REFERENCES 196

APPENDIX 1: GC-MS ANALYSIS OF BIO-OIL FROM CONTINUOUS REACTORS 207

APPENDIX 2: GC-MS ANALYSIS FROM PY-GC-MS 215

APPENDIX 3: EQUATIONS FOR GASIFICATION 218

APPENDIX 4: PUBLICATIONS 220

Publications 220

Submitted and Awaiting Review 220

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LIST OF FIGURES Figure 1: Overall Dibanet process (adapted from [3, 4]) .............................................. 16

Figure 2: Biomass components (adapted from [7]) ...................................................... 20

Figure 3 : DTG profiles of biomass components [14] .................................................. 21

Figure 4: Chopped miscanthus ................................................................................... 22

Figure 5: Miscanthus pellets ....................................................................................... 22

Figure 6: Sugarcane bagasse ..................................................................................... 24

Figure 7: Sugarcane bagasse pellets (as received) .................................................... 24

Figure 8: Sugarcane trash (as received) ..................................................................... 24

Figure 9: Overall acid hydrolysis process .................................................................... 25

Figure 10: DTG profiles of biomass components, biomass and AHR [37] ................... 26

Figure 11: Untreated ground beech ............................................................................ 28

Figure 12: AHR from ground beech: ........................................................................... 28

Figure 13: Untreated sugarcane bagasse (as received) .............................................. 28

Figure 14: AHR from sugarcane bagasse: 5 wt.% H2SO4, 1hour, 175°C (as received) 28

Figure 15: Lignocellulosic component analysis of biomass (adapted from [38]) .......... 30

Figure 16: SEM of miscanthus .................................................................................... 31

Figure 17: SEM of AHR derived from miscanthus ....................................................... 31

Figure 18: SEM of sugarcane bagasse Figure 19: SEM of bagasse pellets .............. 32

Figure 20: SEM of sugarcane trash ............................................................................ 32

Figure 21: Ash content ( wt.%) of beech and AHR derived from beech ....................... 34

Figure 22: HHV (MJ/kg) of beech and AHR derived from beech ................................. 35

Figure 23: Ash content (wt.%) of miscanthus and AHR derived from miscanthus ...... 36

Figure 24: Ash content (wt.%) of sugarcane waste and AHR derived from sugarcane

bagasse ...................................................................................................... 37

Figure 25: Calculated HHV (MJ/kg) of miscanthus and AHR derived from miscanthus 37

Figure 26: Calculated HHV (MJ/kg) of sugarcane waste and AHR derived from

sugarcane bagasse .................................................................................... 38

Figure 27: DTG profiles of beech and AHR derived from beech .................................. 41

Figure 28: DTG profiles of miscanthus and AHR derived from miscanthus ................. 42

Figure 29: DTG profiles of sugarcane waste and AHR from sugarcane bagasse ........ 43

Figure 30: Comparing DTG profiles of AHR from miscanthus and sugarcane bagasse

................................................................................................................... 43

Figure 31: Effect of particle size on ash content (wt.%) ............................................... 46

Figure 32: Alcell lignin and AHR after melt test at 200°C ............................................ 48




Figure 36: AHR after melt test at 600°C ...................................................................... 48

Figure 37: Biomass thermal conversion processes (derived from [6, 60, 61]) ............. 51

Figure 38: Fast pyrolysis process (adapted from [64]) ................................................ 55

Figure 39: Applications of Bio-oil (adapted from [76]) .................................................. 56

Figure 40: Biomass pyrolysis (adapted from [80]) ....................................................... 59

Figure 41: Degradation products from hemicellulose [82] ........................................... 59

Figure 42: Degradation products from cellulose [82] ................................................... 60

Figure 43: Degradation products from lignin [82]......................................................... 60

Figure 44: Bubbling fluidised bed [83] ......................................................................... 61

Figure 45: Circulating fluidised bed/transporting reactor [83] ...................................... 61

Figure 46: Rotating cone pyrolyser [83] ...................................................................... 61

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Figure 47: Ablative pyrolyser [83] ................................................................................ 61

Figure 48: Geldart’s classification of powders according to fluidisation properties [91] 63

Figure 49: Effect of temperature on product yields from fast pyrolysis of wood (wt.% on

dry feed basis) [97] ..................................................................................... 65

Figure 50: Catalytically cracking of fast pyrolysis products (adapted from [73]) ........... 74

Figure 51: Integrated catalyst pyrolysis (adapted from [142]) ...................................... 74

Figure 52: Close coupled secondary fixed bed catalytic reactor .................................. 75

Figure 53: Sequential fixed bed catalytic reactors ....................................................... 75

Figure 54: Sequential catalytic upgrading ................................................................... 76

Figure 55: Flowsheet of 100g/h continuous fluid bed reaction system ......................... 80

Figure 56: 100g/h continuous fluid bed reaction system.............................................. 81

Figure 57: Flowsheet of the 300g/h rig with interchangeable collection systems ......... 85

Figure 58: Glassware collection system ...................................................................... 86

Figure 59: 300g/h rig with quench collection system ................................................... 86

Figure 60: Flowsheet of the 1kg/h continuous fluidised bed reaction system .............. 90

Figure 61: 1kg/h continuous fluidised bed reactor system ........................................... 91

Figure 62: Char in the end of the glass transition pipe ................................................ 94

Figure 63: Phase separation in the main oil collection pot ........................................... 94

Figure 64: Feed backing up in the clear chamber of the feeder ................................... 97

Figure 65: Feed backing up in the vertical feeding tube between the metering screw

and the high speed feed screw in test 8. ..................................................... 97

Figure 66: Second cyclone with char blocking the pipe work ...................................... 97

Figure 67: Example of char retained in the reactor ...................................................... 98

Figure 68: Example of high water content phase-separated bio-oil ............................. 98

Figure 69: Bio-oil collected in oil pot 1, 2 and 3 ......................................................... 104

Figure 70: Bio-oil from miscanthus ............................................................................ 105

Figure 71: Bio-oil from sugarcane bagasse ............................................................... 105

Figure 72: Bio-oil from sugarcane trash .................................................................... 105

Figure 73: Bio-oil from sugarcane bagasse pellets .................................................... 105

Figure 74: pH comparison of main bio-oil and condensates on the 300g/h rig and

glassware collection system ...................................................................... 106

Figure 75: Organics on wall of ESP and main oil pot on the glassware collection system

................................................................................................................. 110

Figure 76: Deposits on the bottom of the ESP on the quench column ....................... 110

Figure 77: Phase separated Isopar and bio-oil from quench column ......................... 111

Figure 78: Effect of rig configuration on product distribution ...................................... 114

Figure 79: pH comparison of main bio-oil and condensates from 3 different rig set ups

................................................................................................................. 115

Figure 80: Variation of the pyrolysis product composition with addition of catalyst .... 121

Figure 81: Effect of catalyst concentration on organic and water content .................. 123

Figure 82: Effect of catalyst concentration on bio-oil composition ............................. 124

Figure 83: Variation of the bio-oil composition as a function of catalyst amount ........ 125

Figure 84: Biomass gasification process ................................................................... 127

Figure 85: Energy flow in and out of a gasifier (adapted from [156]) ......................... 128

Figure 86: Applications of product gas from gasification (adapted from [66]) ............ 129

Figure 87: Status of gasification technologies (redrawn from [170]) .......................... 134

Figure 88: Reaction mechanism of biomass gasification [174] .................................. 136

Figure 89: Gasification furnace at Cardiff University ................................................. 146

Figure 90: Batch system set up for gasification ......................................................... 146

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Figure 91: End of reactor .......................................................................................... 146

Figure 92: Removable reactor end pipe .................................................................... 146

Figure 93: Liquid collection system ........................................................................... 147

Figure 94: AHR blown out of reactor into the ESP .................................................... 150

Figure 95: Gas yields from pyrolytic gasification of SCBP and AHR up to 650°C ...... 151

Figure 96: H2/CO ratios of gaseous products ............................................................ 152

Figure 97: H2/CO2 ratios of gaseous products ........................................................... 152

Figure 98: CO/CO2 ratios of gaseous products ......................................................... 152

Figure 99: Product yields from pyrolytic gasification of SCB and AHR ...................... 154

Figure 100: Comparison of product yields from SCBP using continuous and batch

gasifiers .................................................................................................... 155

Figure 101: Comparison of product yields from AHR using continuous and batch

gasifiers .................................................................................................... 155

Figure 102: Effect of air addition on product yields from batch gasification using SCBP

................................................................................................................. 156

Figure 103: Effect of air addition on product yields from batch gasification using AHR

................................................................................................................. 156

Figure 104: Changing concentration of CO and CO2 from AHR ................................ 157

Figure 105: Effect of initial reactor temperature on time taken for CO to peak .......... 157

Figure 106: Effect of initial reactor temperature on the HHV of oxygen-free product gas

................................................................................................................. 158

Figure 107: Effect of temperature on equivalence ratios ........................................... 159

Figure 108: Composition of product gas from SCBP (vol.%) ..................................... 159

Figure 109: Composition of product gas from AHR (vol.%) ....................................... 160

Figure 110: Effect of initial reactor temperature on solid residue yield ....................... 162

Figure 111: SCBP ..................................................................................................... 163

Figure 112: Gasified SCBP at 950°C ........................................................................ 163

Figure 113: AHR ....................................................................................................... 163

Figure 114: Gasified AHR at 950°C .......................................................................... 163

Figure 115: Effect of temperature on cold gas efficiency and carbon conversion

efficiency of AHR gasification .................................................................... 164

Figure 116: Iso-propanol before run .......................................................................... 165

Figure 117: Iso-propanol after SCBP run .................................................................. 165

Figure 118: Iso-propanol after AHR run .................................................................... 165

Figure 119: Effect of initial reactor temperature on tar yield ...................................... 165

Figure 120: Effect of initial reactor temperature on gas yield ..................................... 166

Figure 121: SCB ....................................................................................................... 171

Figure 122: SCBP ..................................................................................................... 171

Figure 123: Modified paste feeding system ............................................................... 172

Figure 124: KAHL Pellet Fracture Strength Tester .................................................... 174

Figure 125: Needle-like AHR on the left and homogenous powder AHR on the right 174

Figure 126: Alcell lignin with methanol (2:1) paste .................................................... 175

Figure 127: AHR acts as a filter cake for methanol ................................................... 175

Figure 128: Methanol filtrate is forced through the solid AHR ................................... 175

Figure 129: Mud-like paste in the bottom of feeder ................................................... 176

Figure 130: Rubber bung to act as a press on the paste and avoid nitrogen leaving the

feeder ....................................................................................................... 176

Figure 131: Thread-like feeding which could not be controlled and feeding too fast .. 176

Figure 132: 1g of bio-oil ............................................................................................ 177

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Figure 133: Bio-oil after curing in the oven at 150°C for 1 hour ................................. 177

Figure 134: Effect of bio-oil addition on volatile and char content .............................. 178

Figure 135: DTG profiles of ground cured pellets ...................................................... 178

Figure 136: Mechanically pressed SCB pellet (as received) ..................................... 179

Figure 137: SCB (100/0/0) ........................................................................................ 180

Figure 138: SCB (75/0/25) ........................................................................................ 180

Figure 139: SCB (75/25/0) ........................................................................................ 180

Figure 140: Effect of adding water and bio-oil to SCB in comparison with mechanically

pressed SCB pellets ................................................................................. 180

Figure 141: AHR (75/0/25) ........................................................................................ 181

Figure 142: AHR (75/25/0) ........................................................................................ 181

Figure 143: Effect of adding bio-oil to AHR for pelletisation compared with adding water

on pellet fracture strength ......................................................................... 181

Figure 144: AHR (100/0/0) ........................................................................................ 182

Figure 145: AHR (90/10/0) ........................................................................................ 182

Figure 146: AHR (80/20/0) ........................................................................................ 182

Figure 147: AHR (70/30/0) ........................................................................................ 182

Figure 148: AHR (60/40/0) ........................................................................................ 182

Figure 149: AHR (50/50/0) ........................................................................................ 182

Figure 150: Effect of bio-oil concentration on AHR pellet fracture strength ............... 183

Figure 151: Ash content of cured AHR pellet with increasing concentration of bio-oil 183

Figure 152: Phase separation when water is added to bio-oil at a 1:1 mass ratio ..... 184

Figure 153: AHR (60/20/20) Method C - 8mm ........................................................... 184

Figure 154: AHR (60/20/20) Method D - 8mm ........................................................... 184

Figure 155: AHR (60/20/20) Method C - 12.5mm ...................................................... 184

Figure 156: Pellets formed using bio-oil compared with using bio-oil and water ........ 185

Figure 157: GC-MS chromatograms of bio-oil from miscanthus and miscanthus pellets

................................................................................................................. 208

Figure 158: GC-MS chromatograms of bio-oil from sugarcane bagasse, sugarcane

bagasse pellets and trash ......................................................................... 210

Figure 159: GC-MS chromatograms of bio-oil from sugarcane bagasse pellets using

three rig configurations ............................................................................. 212

Figure 160: Comparative chromatograms from Py-GC-MS of SCBP with and without

molybdenum carbide................................................................................. 215

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LIST OF TABLES Table 1: Aims and objectives of this work ................................................................... 17

Table 2: Details of AHR .............................................................................................. 27

Table 3: Approximate lignocellulosic composition (wt.% dry basis) of tested feedstocks

................................................................................................................... 31

Table 4: Sulphur and Chlorine content of biomass tested [49] .................................... 32

Table 5: Elemental analysis for beech (dry basis) ....................................................... 33

Table 6: Elemental composition comparison of humins and lignin .............................. 33

Table 7: Elemental analysis of Dibanet samples (dry basis) ....................................... 36

Table 8: TGA results of beech and AHR derived from beech (dry basis) .................... 39

Table 9: TGA results of Dibanet samples (dry basis) .................................................. 40

Table 10: Comparison of ash content using the ASTM standard and TGA ................. 41

Table 11: Bulk density of feedstocks, sand and char .................................................. 47

Table 12: Typical product yields (dry wood basis) obtained by different pyrolysis modes

[64] ............................................................................................................. 52

Table 13: Comparison of typical product yields (dry wood basis) from gasification [64,

66] .............................................................................................................. 52

Table 14: Advantages and disadvantages of combustion, fast pyrolysis and gasification

[68, 69] ....................................................................................................... 53

Table 15: Comparison of fast pyrolysis and gasification .............................................. 54

Table 16: Unwanted characteristics of Bio-oil (adapted from [73, 77]) ......................... 57

Table 17: Comparison of typical bio-oil and diesel characteristics (derived from [78, 79])

................................................................................................................... 58

Table 18: Advantages and disadvantages of a fluid bed adapted from [84-88] ........... 62

Table 19: Fluidisation regimes (adapted from [93]) ..................................................... 64

Table 20: Advantages and disadvantages of bio-oil upgrading techniques (derived from

[143]) .......................................................................................................... 73

Table 21: Advantages and disadvantages of various catalyst incorporations .............. 77

Table 22: Strengths and weaknesses of reaction systems available at BERG ............ 79

Table 23: Proposed solutions to problems on the quench column .............................. 88

Table 24: Test summary for fast pyrolysis of biomass ................................................. 95

Table 25: Fluidising velocities for Test 11, 16, 12 and 17 ............................................ 98

Table 26: Fast pyrolysis operating conditions for successful tests .............................. 99

Table 27: Mass balance summary from fast pyrolysis of beech using 100g/h rig (dry

feed basis) ................................................................................................ 100

Table 28: Molecular weight distribution of main bio-oil from beech on the 100g/h rig 101

Table 29: Analysis of bio-oil and char from beech ..................................................... 101

Table 30: Mass balance summaries for the 300g/h rig and glassware collection system

................................................................................................................. 103

Table 31: Molecular weight distribution of feedstocks ............................................... 106

Table 32: Analysis of bio-oil from feedstocks ............................................................ 107

Table 33: Analysis of char from feedstocks ............................................................... 107

Table 34: Screening summary of feedstocks ............................................................ 108

Table 35: Reproducibility results for ground SCBP ................................................... 108

Table 36: Comparing mass balance summaries for fast pyrolysis of miscanthus and

SCBP on the 300g/h rig (dry feed basis) ................................................... 109

Table 37: Advantages and disadvantages of the collection systems ......................... 111

Table 38: Comparing fast pyrolysis product yields from SCBP on three rigs ............. 113

Table 39: Molecular weight distribution of main bio-oil from SCBP ........................... 114

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Table 40: Water content and viscosity of main bio-oil from SCBP ............................. 115

Table 41: Analysis of bio-oil from SCBP ................................................................... 115

Table 42: Analysis of char from SCBP ...................................................................... 116

Table 43: Comparison of product yields from fast pyrolysis of SCBP, beech and wood

................................................................................................................. 117

Table 44: Sand, catalyst, feedstock and char bulk densities ..................................... 121

Table 45: Catalytic pyrolysis of ground SCBP at 500°C ............................................ 122

Table 46: Viscosity and pH analysis of bio-oil and secondary condensates from

catalytic pyrolysis of SCBP with Mo2C ...................................................... 123

Table 47: Elemental analysis of bio-oil from catalytic pyrolysis of SCBP with Mo2C .. 124

Table 48: GPC data of bio-oil from catalytic pyrolysis of SCBP with Mo2C ................ 125

Table 49: Advantages and disadvantages of various gasifiers (derived from [156, 164-

167]) ......................................................................................................... 131

Table 50: Typical gas composition from different gasifier types [62].......................... 133

Table 51: Typical gas compositions from biomass [158, 159, 164, 171, 172] ............ 135

Table 52: Gasification reactions [16, 157] ................................................................. 135

Table 53: Contaminants present in product gas (derived from [171, 189]) ................ 140

Table 54: Compounds in liquid products from pyrolysis and gasification [62] ............ 140

Table 55: Reported advantages and disadvantages of catalysts used for tar removal

[163] [194] [156, 195] ................................................................................ 141

Table 56: Typical equipment for gas cleaning [163, 171] .......................................... 142

Table 57: Measuring mass loss due to evaporation of iso-propanol at 950°C ........... 147

Table 58: Gasification test summary ......................................................................... 149

Table 59: Changes to experiment to overcome AHR feeding problems .................... 150

Table 60: Mass balances for pyrolytic gasification of SCB pellets and AHR .............. 153

Table 61: Comparison of gas composition (vol%) with commercial fixed bed downdraft

gasifiers .................................................................................................... 161

Table 62: Solid residue appearance after gasification ............................................... 162

Table 63: Mass balance from air-blown batch gasification ........................................ 166

Table 64: Preparation methods and composition of pellets ....................................... 173

Table 65: AHR and BTG bio-oil ultimate analysis...................................................... 177

Table 66: Ultimate analysis of pellets produced after curing AHR and bio-oil mixtures in

the oven at 150°C for 1 hour ..................................................................... 179

Table 67: Conclusions match to the aims and objectives of this work ....................... 188

Table 68: Liquid composition of bio-oil from beech ................................................... 207

Table 69: Liquid composition of bio-oil from miscanthus and miscanthus pellets ...... 209

Table 70: Liquid composition of bio-oil from sugarcane bagasse, sugarcane bagasse

pellets and trash ....................................................................................... 211

Table 71: Liquid composition of bio-oil from sugarcane bagasse pellets using three rig

configurations ........................................................................................... 213

Table 72: Compound peak area % of total peak area ............................................... 214

Table 73: Quantification of product groups using peak area % ................................. 215

Table 74: Compound identification and quantification ............................................... 216

14

ABBREVIATIONS

AHR Acid Hydrolysis Residue

ASTM American Society for Testing and Materials

BERG Bioenergy Research Group

BTG Biomass Technology Group

CTC Cane Technology Centre

Daf Dry ash free

Dibanet Development of Integrated Biomass Approaches Network

DMB Diesel Miscible Biofuel

DTG Differential Thermogravimetric

EC European Commission

ER Equivalence Ratio

ESP Electrostatic Precipitator

GC Gas Chromatography

GC-MS Gas Chromatography- Mass Spectroscopy

GPC Gel Permeation Chromatography

HHV Higher Heating Value

PDI Polydispersity Index

Py-GC-MS Pyrolysis- Gas Chromatography- Mass Spectroscopy

SCBP Sugarcane Bagasse Pellets

SEM Scanning Electron Microscopy

TGA Thermogravimetric Analysis

UL University of Limerick

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1 INTRODUCTION

Increasing worldwide energy demands have resulted in the increased

dependency on fossil fuels such as coal, gas and crude oil. Significant interest in

alternative sustainable solutions has been generated across the world to meet energy

requirements and reduce carbon dioxide emissions. Biomass is recognised as a unique

renewable energy resource that fixes atmospheric carbon dioxide. Biomass conversion

processes and product upgrading technologies are extensively being developed to

produce bioenergy and biofuels which could initially supplement and eventually replace

fossil fuel derived energy and fuels [1].

1.1 Dibanet project overview

The research in this thesis was carried out for the European Commission

sponsored Dibanet (Development of Integrated Biomass Approaches Network) project

which primarily aimed to produce sustainable diesel miscible biofuels (DMB) from

wastes and residues to improve renewability of transportation fuels by replacing diesel

with biofuels. Agricultural residues such as sugarcane bagasse, which do not compete

with food, can potentially be utilised more efficiently to create sustainable second

generation biofuels. The project also aimed to increase collaboration between Europe

and Latin America.

1.2 Dibanet scientific research objectives

The project co-ordinators of the Dibanet project focussed their efforts on the

production of levulinic acid by acid hydrolysis of biomass. Levulinic acid was

subsequently esterified with ethanol to produce ethyl levulinate for use as a diesel

miscible biofuel. This acid hydrolysis process uses conditions which simulate the

BioFine process and leaves a substantial amount of high lignin and humin content solid

acid hydrolysis residue (AHR) [2]. AHR represents up to 80% of the original energy in

the biomass and therefore is a potentially valuable feedstock for further processing.

One of the aims of the Dibanet project was to evaluate the potential for

converting AHR into higher value products, such as a usable fuel for electricity

generation and potentially DMB, by fast pyrolysis and gasification. Fast pyrolysis and

gasification were considered as they are reported to produce high liquid and gas yields,

respectively. However, further upgrading is required in both cases. Another aim was to

integrate processes to make processes energetically self-sufficient by firstly utilising the

AHR and then by thermally processing additional biomass if the energy from the AHR

was insufficient. Therefore, both biomass and AHR were tested in this work. The

overall process is shown in Figure 1.

16

Figure 1: Overall Dibanet process (adapted from [3, 4])

Biomass Acid hydrolysis

Solid AHR

Pyrolysis Upgrading Diesel miscible

biofuel

Gasification Gas

cleaning Synfuels or

heat & power

Liquid containing levulinic acid,

formic acid and furfural

Ethyl levulinate

Formic acid and fufural

17

1.3 Scientific research objectives

The overall objective of the research reported in this thesis was to explore the

potential of producing DMB and/or bioenergy through pyrolysis and gasification of both

AHR and biomass. Table 1 outlines the aims and objectives of this work and the

approach adopted in order address these aims and objectives.

Table 1: Aims and objectives of this work

Aims and Objectives Approach

Evaluate the composition and properties of biomass and AHR. Compare AHR with other lignin materials and investigate the effect of

humins.

Using literature

Scanning Electron Microscopy (SEM)

Proximate and ultimate analysis

Thermogravimetric analysis (TGA)

Bulk density testing

Melt testing and comparisons with Alcell lignin

Investigate fast pyrolysis of biomass and AHR in order to produce liquid bio-oil

Using literature

By TGA

If results are promising from TGA, process AHR on a bench-scale fast pyrolysis rig

Compare miscanthus, miscanthus pellets, sugarcane bagasse, sugarcane bagasse

pellets sugarcane trash. Test most promising feedstock on fast pyrolysis rigs at Aston University. Suggest methods to improve

mass balances and compare product yields using different liquid collection systems.

Prepare biomass according to required particle size

Test on a bench-scale fast pyrolysis rig by overcoming feeding and fluidisation problems.

Determine product yields from each feedstock.

Select most promising feedstock with highest organic liquid yield from smaller scale bench-scale processing.

Compare product yields and properties from this feedstock on 2 rigs with 3 liquid collection systems.

Compare this feedstock to beech wood.

Investigate the effect of molybdenum carbide on pyrolysis

Investigate the effect of adding molybdenum carbide to a bench-scale fast pyrolysis unit. Analyse liquid product for water content, composition, viscosity, homogeneity and pH.

Understand the effect of process and feedstock variables on fast pyrolysis and

gasification

Using literature

Compare Dibanet feedstocks

Investigate the effect of temperature on fast pyrolysis.

Investigate gasification of AHR with biomass in order to produce a usable gas for heat and

power or potentially synfuels

Pyrolytic gasification

Air-blown gasification

Improve feeding and handling properties of AHR. Evaluate screw feeding, paste feeding and pelletisation of AHR. Investigate whether

bio-oil can be used as a binder in pelletisation of AHR. If so, determine the

minimum about of bio-oil required for successful pelletisation.

Screw feed tests on bench-scale fast pyrolysis units

Past feeding with methanol

Pelletisation of AHR with water

Pelletisation of AHR with increasing concentrations of bio-oil

18

1.4 Structure of thesis

This thesis extends over twelve chapters and the structure is described below.

Chapter one provides an overview and the detailed scientific research objectives

of the Dibanet project. This chapter also outlines the structure of the thesis.

Chapter two describes the biomass and acid hydrolysis residues tested in this

work.

Chapter three describes the characterisation techniques employed to analyse the

biomass and acid hydrolysis residues tested in this work. Results are also

presented and discussed.

Chapter four describes and compares the available thermal conversion

processes.

Chapter five looks at the theory and literature review of fast pyrolysis and bio-oil

upgrading.

Chapter six describes and compares the fast pyrolysis of biomass in three sizes

of fluidised beds using alternative liquid collection systems. Product analysis

techniques used to analyse solid, liquid and gaseous products are also

described. Careful mass balances were carried out, closures are reported and

means of improvement are suggested.

Chapter seven presents the effect of novel molybdenum carbide on pyrolysis of

sugarcane bagasse using both Py-GC-MS and a fluidised bed.

Chapter eight focuses on theory and literature review of gasification and gas

upgrading.

Chapter nine is the gasification section which compares pyrolytic gasification on a

continuous unit at Aston University and air-blown gasification on a batch unit at

Cardiff University. Limitations with the two gasification system are also reported.

The effect of gasification temperature on product yields and composition from

biomass and AHR is also compared.

Chapter ten describes the methods used to overcome feeding problems of acid

hydrolysis residue powder. It includes details of unsuccessful paste feeding tests

and successful pelletisation tests using bio-oil.

Chapter eleven recaps the interim conclusions presented at the end of each

chapter.

Chapter twelve makes recommendations for future research.

19

2 BIOMASS AND ACID HYDROLYSIS RESIDUES TYPES

The objective of this chapter is to examine the background of biomass, biomass

components and the fifteen feedstocks tested in this work. Six biomass and nine acid

hydrolysis residues (AHR) are also analysed and characterised by standard methods to

identify a suitable thermal conversion process for the production of bioenergy and/or

biofuels.

2.1 Biomass

Biomass includes agricultural and forestry wastes (wood chips, straw), industrial

and consumer waste. It is a unique source of renewable energy as it produces fixed

carbon from atmospheric CO2 through photosynthesis. Also, woody biomass has a

relatively low ash, sulphur and nitrogen content compared to coal. In this context, ash

refers to the inorganic content after ashing or combustion and follows the conventional

presentation of biomass characteristics. On the downside, large amounts of land are

required for growing energy crops and there are cost issues with storage and transport

of biomass. However, using waste and residues overcomes the land use issue. Fuel

vs. food issues associated with first generation biomass are also overcome. Converting

wastes and residues into a transportable liquid or gaseous fuel is a more acceptable

way of producing second generation fuels and chemicals. For example, instead of

using sugar to produce first generation bioethanol, waste sugarcane bagasse could be

utilised to produce second generation biofuels which does not directly compete with

food.

2.1.1 Biomass components

Biomass composition can vary depending on biomass type and origin. Land

based biomass is lignocellulosic material made up of cellulose, hemicellulose and

lignin. Biomass also contains small amounts of extractives and alkali metals in the form

of ash, some of which are catalytically active. Biomass also contains moisture,

nitrogen, phosphorus, chlorine and sulphur. Chlorine and sulphur are released as gas

or are bound within the ash and in order to satisfy emission limits, chlorine and sulphur

should be minimised. ECN have reported that the sulphur content in biomass range

from 0.05 wt.% to more than 3 wt.%. Chlorine content of biomass is also reported to

range from 0.01 wt.% to 2.4 wt.% on dry and ash free basis [5].

Figure 2 shows the main components present in biomass and shows

approximate proportions of cellulose, hemi-cellulose and lignin as reported by Goyal et

al. [6].

20

Figure 2: Biomass components (adapted from [7])

Extractives act as energy reserves and as defence against microbial and insect

attack [7]. Examples include fats, waxes, alkaloids, proteins, phenolics, simple sugars,

gums, resins, starches and essential oils [7]. Polar solvents (such as water, methylene

chloride or alcohol) or nonpolar solvents (such as toluene or hexane) can be used to

extract these extractives from biomass [7]. Acid hydrolysis uses water in the treatment

process and therefore, water soluble extractives are removed from acid hydrolysis

residues. Sugarcane is also washed with warm water in the sugar recovery process,

therefore sugarcane bagasse is expected to be substantially water soluble extractive-

free.

Biomass contains trace amounts of ash which consist of inorganic materials

such as potassium, sodium, phosphorus, calcium and magnesium [7]. Ash is

undesirable in fast pyrolysis because catalytically active alkali metals, present in the

ash, are responsible for secondary cracking reactions which influence the

decomposition products [8]. However, catalytically active ash cracks undesirable tars in

gasification. Agricultural residues and grasses are reported to have higher ash content

than wood [9] and ash analysis is carried out in section 3.1 to support this.

Cellulose consists of long polymer chains made up of D-glucose subunits,

linked by b-1,4 glycosidic bonds. Cellulose gives biomass a tough fibrous nature and is

made up of an arranged crystalline structure and non-arranged amorphous region [10].

Acid hydrolysis of biomass leads to oligomerisation of these long polymer chains

resulting in the refractory humins found in AHR. Humins are described later in section

2.2.

Hemicellulose is made of short lateral polymer chains like five-carbon

(pentoses), six carbon (hexoses) sugars and sugar acid. Hemicellulose can be more

easily hydrolysed than cellulose and is stated to be the most thermal-chemical

sensitive compared to cellulose and lignin. Hemicellulose acts as a connection

Biomass

Low MW substances

Organics Extractives

Inorganics Ash

High MW substances

Polysaccharides

Cellulose

(40-50 wt.%)

Hemicellulose

(25-40 wt.%) Lignin

(15-30 wt.%)

21

between cellulose and lignin and gives the biomass structure more rigidity [10], in other

words, it holds the cellulose bundles together.

Lignin is an amorphous heteropolymer made up of phenylpropane units (p-

coumaryl, coniferyl and sinapyl alcohol). Lignin is reported to contain nearly 60 wt.%

carbon [11]. After cellulose and hemicellulose, lignin is one of nature’s most abundant

polymers [3]. It provides “structural support, impermeability and resistance against

microbial attack and oxidative stress” [10]. Lignin is reported to be the most thermally

resistant component in biomass and the method of lignin isolation has significant effect

on its structure and thermal decomposition properties [12]. Lignin is widely available as

a by-product from many processes such as second generation ethanol biorefineries.

Dissolution of lignin into a solution, such as the Kraft pulping process or hydrolysis of

cellulose and hemicellulose by acid, leaving lignin as an insoluble residue, are two of

the methods of extracting lignin from lignocellulosic biomass [13].

The thermal degradation properties of biomass are strongly dependent on the

lignocellulosic composition and catalytic activity of ash. Figure 3 shows the Differential

thermogravimetric (DTG) profiles of biomass components. Xylan is representative of

hemicellulose and is the least refractory of the biomass components as the maximum

rate of devolatilisation is at approximately 300°C. The maximum rate of devolatilisation

for cellulose is at approximately 350°C. It can be seen that the DTG profile for lignin is

flatter indicating that lignin is more refractory than holocellulose and decomposes over

a wider range of temperatures.

Figure 3 : DTG profiles of biomass components [14]

22

2.1.2 Biomass tested

Six biomass samples were analysed and processed in this work. Beech was

used as a reference material early in this research when samples from the Dibanet

project were not available. Miscanthus and miscanthus pellets were the primary

European feedstock and sugarcane bagasse, sugarcane bagasse pellets and

sugarcane trash were used as the Latin American feedstocks within the Dibanet

project.

Beech wood, with a very low ash content, has extensive data available from

other work so can be used for cross checking results and as a reference material.

Beech was supplied by Rettenmeier based in Germany.

Miscanthus x Giganteus is a fast growing C4 perennial woody type grass.

Miscanthus has a low nutrient requirement and is harvested annually in March/April in

Europe while the plant is senescent to reduce the ash and moisture contents and leaf

to stem ratios. Miscanthus is an energy crop which is commercially grown to produce

fuel. Yields of this energy crop are reported to range from 27 to 44 wet tonnes/ha/year

in Europe [15]. Miscanthus (see Figure 4) was sourced from JHM Crops Ltd.

(www.jhmcrops.ie), in Ireland. The miscanthus was chopped to 2cm, but not dried.

Pellets can be made from biomass to increase the bulk and energy density.

Utilising pellets can increase throughputs in a continuous process and also allow more

efficient and cost effective transport, handling and storage. Pellets are also more

uniform and create less fines [16]. Miscanthus pellets with an 8mm diameter (see

Figure 5) were supplied by Ignite Wood Fuels LTD, UK. Miscanthus pellets were

ground for analysis and processing in this work. Explanations for grinding pellets can

be found in section 3.3.

Figure 4: Chopped miscanthus

(as received)

Figure 5: Miscanthus pellets

(as received)

https://mail.aston.ac.uk/owa/redir.aspx?C=44a34e00b40b49f48957314823b13865&URL=http%3a%2f%2fwww.jhmcrops.ie

23

Sugarcane is a perennial C4 plant which is harvested annually between April

and December (in Brazil). The main product of sugarcane is sucrose which can be

used in the food industry or fermented to produce bio-ethanol. The stem of the

sugarcane contains the largest quantity of sucrose and so is processed at the sugar

mill soon after the sugarcane has been harvested. The residue from sugar recovery is

a fibrous lignocellulosic residual waste called bagasse. In Brazil, 140kg of dry bagasse

is produced per wet tonne of sugarcane processed [17]. Currently, the bagasse is

recovered for use as a fuel in boilers to produce heat for the sugar mill. It is reported

that these mills deliberately use inefficient burners to utilise the excessive waste and

reduce the need for sending this waste to landfill [17, 18]. However, alternative, more

efficient processes should be used to maximise the energy output from bagasse in

order to reduce the reliance on fossil fuels for heat and power generation [17, 18].

Sugarcane bagasse (see Figure 6) was collected from storage piles at a sugar

mill site in Brazil in August 2010 by members of Cane Technology Centre (CTC). The

moisture content of the bagasse was between 50 and 60 wt.% and dried at room

temperature for two weeks to avoid biological activity and degradation before being

stored in boxes.

Sugarcane bagasse pellets (SCBP) were also used in this work to investigate

the effect of densification on feeding ability and product yields. In order to make SCBP

(shown in Figure 7), bagasse was dried in a flash drier to a moisture content of around

11 to 12 wt.%. The dryer promoted drying by direct contact between a hot exhaust gas

(from a boiler) and bagasse. The continuous drying process used temperatures of

approximately 280°C with a contact time of 3-4 seconds. The dried bagasse was then

ground and pelletised in Brazil with no additives or steam. Pellet production is not

standard practice at sugar mills in Brazil. However, CTC were involved in a small

project testing SCBP and therefore a batch was sent to Aston University in April 2011

for thermal processing tests.

The bagasse drying temperature, similar to that of torrefaction (200 to 300 °C)

leads to a loss of moisture and light organic materials. Torrefaction is a primary thermal

process which occurs in an inert atmosphere between 200-300°C and can be applied

to biomass to leave a dry, hydrophobic material with increased energy density [19].

Torrefaction of high moisture content sugarcane bagasse (50 and 60 wt.%) can also

avoid biological degradation. Figure 7 shows the SCBP which are darker compared to

loose bagasse. The hemicellulose and the fibrous structure of cellulose within biomass

are destructed leaving a brittle and darkened material which can reduce milling costs,

but can be friable during transport. Sugarcane bagasse pellets were ground prior to

analysis and processing and explanations for grinding can be found in section 3.3.

24

Figure 6: Sugarcane bagasse

(as received)

Figure 7: Sugarcane bagasse pellets (as received)

Sugarcane trash comprises the leaves and tops of sugarcane which are

traditionally left on the sugarcane field and set on fire to facilitate sugarcane harvest.

Due to pollution from burning of the cane fields [20], these manual harvesting

techniques are being replaced by mechanical harvesting where a harvester cuts the

crop at the base of the stalk, strips off the leaves and returns the leaves (trash) back to

the field. There is currently a debate on whether trash should be left on fields to return

the nutrients back to the soil or collected to produce bioenergy. It is reported that the

same amount of trash is produced as bagasse. Therefore, 140kg of dry trash is

produced per wet tonne of sugarcane processed [17]. Sugarcane trash (shown in

Figure 8) was manually collected off the fields in Brazil by members of CTC in August

2010 and as with the bagasse, was dried at room temperature for two weeks and

stored in boxes.

Figure 8: Sugarcane trash (as received)

2.2 Acid hydrolysis of biomass

Biomass can be pre-treated before it is thermally processed in order to break up

the lignocellulosic structure. Acid hydrolysis has been extensively investigated to

fractionate the lignocellulosic structure and release soluble sugar and other chemicals

from biomass [21]. Acid treatment can also reduce the metal/ash content in biomass

25

and increases the yield of volatiles [22]. A reduction in ash content increases the

organic liquid yield from fast pyrolysis by reducing catalytic cracking.

Both concentrated and dilute acid hydrolysis has been reported in the literature.

Acid recovery issues and corrosion of acid hydrolysis equipment are limiting factors in

concentrated acid hydrolysis. However, dilute acid hydrolysis has been well developed

[23] where a dilute acid can be used as a liquid phase catalyst to pre-treat

lignocellulosic materials. Sulphuric acid is commonly employed for acid hydrolysis [23-

29] and has been used to manufacture furfural [21, 29]. However, other researchers

have also investigated acid hydrolysis using hydrochloric [23, 30, 31], phosphoric [32,

33] and nitric acid [34]. Hydrochloric acid is viewed as unsuitable for pre-treating

biomass as traces of chlorine can act as a catalyst and have undesirable effects in

pyrolysis.

Part of the Dibanet project involved the production of levulinic acid by acid

hydrolysis of biomass. Figure 9 outlines the overall acid hydrolysis process where

biomass components are converted into water soluble chemicals containing a mixture

of levulinic acid, formic acid and furfural. The composition of this mixture can be varied

according to the process parameters employed.

Figure 9: Overall acid hydrolysis process

2.2.1 Acid hydrolysis residues components

The remaining solid acid hydrolysis residues from the Dibanet project mainly

consist of lignin and humins. Humins are carbonaceous dark coloured solids and are

insoluble polymeric materials which are spherical in shape and have an aromatic

Biomass

Lignin

Hemicellulose

Cellulose

Hexose

Pentose

Acid

hydrolysis

residues

Glucose5-Hydroxy-

methylfurfuralLevulinic acid +

Formic acid

Mannose

Glucose

Galactose

Xylose

Arabinose

Furfural

Acid hydrolysis

Thermal

processingUpgrading

Diesel Miscible

Biofuel

26

character [35] [3] [36]. Oligomerisation of the long cellulosic polymer chains results in

the refractory and undesirable humins found in AHR. Humins are also reported to be

derived from glucose and 5-Hydroxy-methylfurfural [36]. Therefore, the formation of

humins is reported to limit levulinic acid yields [36]. Researchers at University of

Limerick (UL) used sulphuric acid to hydrolyse miscanthus and sugarcane bagasse.

However, sulphur in sulphuric acid can deactivate catalysts if they remain in AHR and

so AHR were thoroughly washed with excessive amounts of water. These AHR were

analysed and thermally processed in this work with the aim of producing biofuels or

heat and power.

Figure 10 compares the DTG profiles of biomass components, miscanthus and

an example of AHR. AHR is shown to decompose at even higher temperatures than

lignin suggesting that the presence of humins makes AHR even more difficult to

thermally decompose. Higher temperatures are required to break the strong bonds

between lignin and humins.

Figure 10: DTG profiles of biomass components, biomass and AHR [37]

2.2.2 Acid hydrolysis residues tested

Three acid treated beech samples were produced at Aston University to

simulate the acid hydrolysis process when AHR samples were not available from UL.

Acid treatment conditions reported in literature were used as acid hydrolysis conditions

were unknown at the beginning of the project. Six AHR were later provided by UL.

Details of all nine AHR are shown in Table 2.

27

Table 2: Details of AHR

Feedstock Conditions Laboratory

AHR from beech Single stage acid treated beech

(1 wt.% H2SO4, 4h, 75°C)

Aston University AHR from beech Two-stage acid treated beech (1 wt.% H2SO4, 4h, 75°C) then

(20 wt.% H2SO4, 4h, 75°C)

AHR from beech One-stage acid treated beech

(20 wt.% H2SO4, 4h, 75°C)

AHR from miscanthus (5 wt.% H2SO4, 2h, 200°C)

University of Limerick



AHR from miscanthus (1 wt.% H2SO4, 10 minutes,

200°C)


AHR from sugarcane bagasse (5 wt.% H2SO4, 1h, 175°C)

Sulphuric acid is reported as the most widely used acid for pre-treatment of

biomass [23-29] and was used for levulinic acid production at UL. Therefore, beech

was subjected to mild acid hydrolysis, using sulphuric acid, to simulate the anticipated

AHR from UL. These beech residues were then used for feeding tests on a continuous

fluidised bed system to identify feeding problems that may occur with AHR from UL.

The mass ratio of beech to sulphuric acid solution was 1:6 (mass basis). Acid

concentration was varied to compare the effect of mild (1 wt.%) and strong (20 wt.%)

acid treatment. Three experiments were carried out using approximately 200g of beech

at the different acid concentrations as shown in Table 2. The particle size used was

355-500 µm to allow pneumatic feeding into a continuous fluidised bed system. The

temperature of the solution was controlled at 75°C +/- 3°C and the system was

maintained at atmospheric pressure. The acid hydrolysis treatment was run for four

hours at 75°C.

Tests were conducted to compare the results of one and two stage strong acid

hydrolysis. One stage acid hydrolysis involved treating the beech in one stage with 20

wt.% acid solution. However, two stage acid hydrolysis involved treating the beech with

1 wt.% acid solution and subsequently treating the remaining solid residue with 20

wt.% solution. The first stage was expected to break down the hemicellulose and the

second stage to break down the cellulose, leaving a high lignin content residue

analogous to residues from levulinic acid production. Figure 12 shows the AHR

produced from ground beech using the most severe acid hydrolysis conditions (20 wt.%

H2SO4, 4 hours, 75°C). Higher lignin and humin content samples are expected to be

darker in colour, therefore as there was very little colour change after acid hydrolysis, it

is expected that significant amounts holocellulose remained in the feedstock.

28

Figure 11: Untreated ground beech

Figure 12: AHR from ground beech:

20 wt.% H2SO4, 4 hours, 75°C

When the 8 litre acid hydrolysis batch reactor was up and running at UL,

miscanthus and sugarcane bagasse (Figure 13) were hydrolysed in dilute sulphuric

acid to maximise levulinic acid production. The effect of sulphuric acid concentration,

reaction time and temperature were investigated by UL and the solid residues

produced were sent to Aston University for analysis and thermal processing. The water

soluble products, including the levulinic acid, from the biomass were collected as a

liquid. The remaining solid AHR, with a moisture content of approximately 75 wt.%, was

filtered and dried at 60°C for at least 12 hours to a moisture content of 6-8 wt.%. Six

AHR were produced at UL using higher temperatures (up to 200°C) and pressures (up

to 15bar), compared to Aston, allowing for lower acid concentrations (maximum of 5%

H2SO4) which produced a solid AHR with significant amounts of holocellulose removed.

Figure 14 shows an example of an AHR from sugarcane bagasse which is

considerably darker than the untreated sugarcane bagasse. It is also much darker than

the AHR produced from beech. This AHR looked similar to the other five AHR from

levulinic acid production.

Figure 13: Untreated sugarcane bagasse (as received)

Figure 14: AHR from sugarcane bagasse: 5 wt.% H2SO4, 1hour,

175°C (as received)

29

AHR from miscanthus and sugarcane bagasse were brown non-homogeneous

fine powders which crumbled easily. Feeding of this material into a pyrolysis or

gasification reactor posed a challenge. The characteristics of the feedstocks described

in this chapter can be found in chapter 3.

2.3 Interim conclusions

Lignin is more refractory than hemicellulose and cellulose and so AHR is more

refractory than whole lignocellulosic biomass.

AHR mainly consists of humins and lignin.

Humins are derived from cellulose.

AHR is shown to decompose at even higher temperatures than lignin suggesting

that the presence of humins makes AHR even more refractory.

AHR produced from beech were insufficiently hydrolysed and did not simulate the

anticipated AHR from UL.

Feeding of powdered AHR into a thermal processing unit is likely to be

problematic.

30

3 BIOMASS AND ACID HYDROLYSIS RESIDUE ANALYSIS

The objective of this chapter is to determine the lignocellulosic composition of

the biomass and acid hydrolysis residues (AHR) tested in this work. Feedstocks are

also characterised by scanning electron microscopy to compare the structure of the

biomass with AHR. Proximate and elemental analysis is also presented to evaluate the

usefulness of the feedstocks as a fuel for thermal processing. Differential

thermogravimetric analysis is also an important factor to investigate feedstock thermal

properties. Preparation methods used for the biomass and the most promising AHR

reported by University of Limerick are subsequently reported.

Figure 15 shows the procedures employed at the beginning of this project to

determine the lignocellulosic composition of biomass and AHR, but the analyses were

unsuccessful because of separation problems due to the finely dispersed nature of the

AHR. It can be seen that although holocellulose and lignin can be separated using the Klason lignin procedure, there is no analytical way of differentiating lignin and humins.

The AHR powder dissolved in the filtrate so mass loss of feedstock was not realistic.

Therefore, results reported in literature were used to compare the lignocellulosic

composition of biomass with AHR.

Figure 15: Lignocellulosic component analysis of biomass (adapted from [38])

Table 3 compares the reported lignocellulosic compositions of the feedstocks

tested in this work. Melligan et al. report the Klason lignin content of AHR as high as

95.5 wt.% [2] and do not make any reference to humins which are expected to be

present in AHR. However, Girisuta et al. reports that the acid insoluble solid AHR

contain unhydrolysed lignin and humins [39] and state that humin content can be

Biomass

Drying Ashing Moisture

content

Ash

content

Dry biomass

Step 1: Extraction

Holocellulose

Klason lignin

Cellulose Hemicellulose

Extractives

Extractive-free biomass

Step 2: Delignification

Dissolution by alkali Calculation

Step 3

Klason lignin in

holocellulose

31

estimated by assuming the lignin content from the original biomass. The calculations

carried out to estimate the composition of the AHR can be found in section 3.2.

Table 3: Approximate lignocellulosic composition (wt.% dry basis) of tested

feedstocks

Feedstock Hemicellulose Cellulose Lignin

Beech [40, 41] 23.51-32.6 38.6-45.1 22.3-26.4

Miscanthus [2, 42, 43] 18-27 37-43 20-25

Sugarcane bagasse [44-46] 23-26 31-52 13-22

Sugarcane trash [47] 26-31 45-48 7-20

AHR [2] 0.1-0.2 0.36-15.4 95.5

Hemicellulose is the most reactive and unstable component of biomass.

However, lignin is the least reactive [48]. For this reason, AHR with a significantly

higher proportion of acid insoluble solids, reported as lignin, is expected to be less

reactive and more refractory compared to whole biomass.

Figure 16 shows the SEM image for miscanthus and Figure 17 shows that the

remaining structure of the AHR derived from miscanthus is still intact; however, the

hemicellulose and cellulose have been removed leaving a very porous structure.

Figure 16: SEM of miscanthus Figure 17: SEM of AHR derived from

miscanthus

The structure of the ground SCBP in Figure 19 is more porous compared to the

loose sugarcane bagasse in Figure 18 suggesting there was loss of material caused by

the drying conditions during pelletisation. Figure 20 shows the SEM image for

sugarcane trash which seems less fibrous than sugarcane bagasse. It is expected that

the washing of sugarcane, in the sugar recovery process, partially destroys the

sugarcane bagasse.

32

Figure 18: SEM of sugarcane bagasse Figure 19: SEM of bagasse pellets

Figure 20: SEM of sugarcane trash

3.1 Proximate and ultimate analysis

Elemental analyses were carried out to calculate the higher heating value of

each feedstock. Feedstocks were dried in an oven, to constant weight, before Carbon

(C), Hydrogen (H) and Nitrogen (N) content analysis was carried out in duplicates using

a Carlo-Erba 1108 elemental analyser by an external company (Medac Ltd.).

Averages of values ( +/- 0.30 wt.%) were used to calculate the higher heating value.

The mean sulphur and chlorine content of the biomass tested in this work, according to

the Phyllis 2 database [49], is presented in Table 4. Sulphur content of these

feedstocks was below the detection level of the equipment used at Medac Ltd. Also,

due to the number of samples and the availability of funding, chlorine analysis was not

possible in this work.

Table 4: Sulphur and Chlorine content of biomass tested [49]

Ash content analyses of samples were carried out in triplicates using the

“Standard Test Method for Ash in Biomass” (ASTM E1755). Averages of results were

taken and the minimum and maximum values are shown as error bars in corresponding

33

figures. Equation 1 was subsequently used to calculate the higher heating value (HHV)

from the elemental and ash data. Channiwala et al. report that this equation can be

used to calculate the HHV of gases, liquids, biomass and residue derived fuels [50].

Equation 1: HHV Equation [50].

HHV = 0.3491C + 1.1783H + 0.1005S - 0.1034O - 0.0151N - 0.0211Ash (MJ/kg)

Table 5 shows the elemental analysis of all of the AHR derived from beech and

the processing conditions are shown in brackets. The nitrogen and sulphur content of

these feedstocks were below the equipment detection limit

34

An increase in carbon content is an indication of increased lignin and humin

content. The carbon content of the AHR from beech was approximately 48 wt.% which

was significantly lower than the carbon content of lignin and humins (60 wt.%) and

more similar to the carbon content of biomass (47.6 wt.%). This suggested that there

were still significant amounts of holocellulose remaining in these AHR. However,

although these AHR were not sufficiently hydrolysed, the carbon content results

indicated that increasing acid concentrations improved holocellulose removal. The

table also shows the oxygen content reduced from 45.62 to 45.19 wt.% due to

increasing carbon content in the AHR.

Figure 21 shows that acid hydrolysis can be used as a pre-treatment method to

reduce the ash content of beech from 0.82 to 0.05 wt.%. One stage acid hydrolysis

reduced the ash content to 0.05 wt.%, but two stage acid hydrolysis reduced the ash

content to 0.08 wt.%. Therefore, one stage acid hydrolysis is preferred if the aim is to

reduce ash content which is catalytically active and undesirable in processes such as

fast pyrolysis.

Figure 21: Ash content ( wt.%) of beech and AHR derived from beech

The reported HHV value of cellulose and hemicelluloses is approximately

18.6MJ/kg. The reported HHV value of lignin ranges from 23.26 to 26.58MJ/kg [53].

The HHV of humins is unknown, but expected to be similar to that of lignin as both

have similar carbon content. Therefore, feedstock HHV is expected to increase with

increasing lignin and humin content. Heating value can be used as an indicator of the

lignin and humin content of a feedstock. Figure 22 shows that the heating value of the

AHR was greater than that of biomass and heating value increased with the severity of

acid hydrolysis conditions as the carbon content increased. The calculated HHV

increased from 18.91 to 19.64MJ/kg by increasing acid treatment from 1 wt.% to 20

wt.%. It is expected that the acid removed some holocellulose which increased the

HHV.

0.00.10.20.30.40.50.60.70.80.9

Beech One-stage acid treatedbeech

(1 wt.% H2SO4)

Two-stage acid treatedbeech

(1 wt.% then 20 wt.%H2SO4)

One-stage acid treatedbeech

(20 wt.% H2SO4)

As

h c

on

ten

t (w

t.%

)

Beech sample

35

Figure 22: HHV (MJ/kg) of beech and AHR derived from beech

Table 7 summarises the elemental data for all eleven Dibanet feedstocks. As

mentioned earlier, an increase in carbon content is an indication of increased lignin and

humin content. The carbon content of the AHR from miscanthus (5 wt.% H2SO4, 1h,

175°C) was the highest 63.21 wt.% which was comparable to the carbon content of

lignin and humins (60 wt.%). This suggested that this AHR had the most holocellulose

removed and these acid hydrolysis conditions were expected to be the optimum for

levulinic acid production. The same acid hydrolysis conditions were applied to

sugarcane bagasse and the carbon content was 62.61 wt.% which was comparable to

the carbon content of the AHR from miscanthus.

The sulphur content was below the analyser detection level (

36

Table 7: Elemental analysis of Dibanet samples (dry basis)

Sample (treatment conditions)

C (wt.%)

H (wt.%)

N (wt.%)

O* (wt.%)

S (wt.%)

Ash (wt.%)

HHV (MJ/kg)

Miscanthus 45.99 6.03 0.49 47.47 Nm 4.94 18.25

Miscanthus pellets 46.82 6.15 0.99 46.02 Nm 4.54 18.82


62.66 4.57 0 32.77 Nm 2.69 23.86


49.66 6.23 0.14 43.97 Nm 0.41 20.13


51.90 6.01 0.15 41.94 Nm 0.38 20.86

AHR from miscanthus (1 wt.% H2SO4, 10 minutes,

200°C) 61.78 5.31 0.515 32.40 Nm 0.93 24.46


63.21 5.01 0.44 29.26 0.16 1.93 24.91

Sugarcane bagasse 47.66 6.06 0.39 45.88 Nm 3.19 19.03

SCBP 43.47 5.66 0.20 44.69 0.27 5.71 17.12

AHR from sugarcane bagasse (5 wt.% H2SO4, 1h, 175°C)

62.61 5.00 0.15 26.07 0.18 6.00 24.94

Sugarcane Trash 45.24 5.88 0.685 48.17 Nm 6.03 17.72

*Oxygen by difference

Figure 23 shows the effect of acid hydrolysis on ash content. The error bars for

ash content each of these feedstocks is also presented. The ash content of all the AHR

were lower than miscanthus. There was little effect of increasing reaction time from 3 to

24 hours. However, the results showed that increasing the reaction temperature from

150 to 200°C increased the ash content from 0.38 wt.% to 0.93 wt.%.

Figure 23: Ash content (wt.%) of miscanthus and AHR derived from miscanthus

Figure 24 shows that the ash content in SCBP was higher (5.71 wt.%) than in

unpelletised bagasse (3.19 wt.%). This could be partly due to possible differences in

biomass source, harvest times, soil contamination or due to the loss of organic

materials from drying or pelletising. Also, storage and handling can affect the

composition of biomass which could explain variances between batches. The ash

content of sugarcane trash is higher than bagasse which is expected to be a result of

soil contamination when the trash was left on the field before collection. The ash

0.0

1.0

2.0

3.0

4.0

5.0

6.0

Miscanthus Miscanthuspellets

AHR (5 wt.%H2SO4, 2h,

200°C)


150°C)


150°C)

AHR (1 wt.%H2SO4, 10

minutes,200°C)


175°C)

As

h c

on

ten

t (w

t.%

)

Miscanthus sample

37

content of the AHR from bagasse did not show a decrease in ash content, but this may

be due to the variances between bagasse batches. Sugarcane bagasse samples

analysed later in the project had an ash content of approximately 20wt.% which

confirms that the ash content of sugarcane bagasse composition can vary significantly

between batches depending on storage location and duration. It is likely that a batch of

bagasse with significantly higher ash (>6 wt.%) was used to make these AHR samples.

Figure 24: Ash content (wt.%) of sugarcane waste and AHR derived from

sugarcane bagasse

Figure 25 shows the calculated HHV of miscanthus compared with AHR from

miscanthus. As expected, increased severity of acid hydrolysis conditions increased

the HHV of AHR due to an increased lignin and humins content which have a higher

energy content (approximately 23.26 to 26.58 MJ/kg) [53] compared to holocellulose

(18.6 MJ/kg) [53].

Figure 25: Calculated HHV (MJ/kg) of miscanthus and AHR derived from

miscanthus

The calculated HHV of sugarcane bagasse was 19.03 MJ/kg compared to AHR

from sugarcane bagasse which was 24.94 MJ/kg (Figure 26). This was also expected

as lignin and humins content increased.

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

Sugarcane bagasse SCBP Sugarcane Trash AHR (5 wt.% H2SO4,1h, 175°C)

As

h c

on

ten

t (w

t.%

)

Sugarcane waste sample

0

5

10

15

20

25

30

Miscanthus Miscanthuspellets


150°C)


150°C)

AHR (1 wt.%H2SO4, 10

minutes,200°C)


175°C)


200°C)

HH

V (

MJ

/kg

)

Miscanthus sample

38

Figure 26: Calculated HHV (MJ/kg) of sugarcane waste and AHR derived from

sugarcane bagasse

Proximate analysis was carried out using Thermogravimetric Analysis (TGA)

where a feedstock was heated up to a specified temperature in a PerkinElmer Pyris 1

thermogravimetric analyser and the mass loss gravimetrically measured. This

technique is not representative of a fast pyrolysis system as it has slower heating rates,

longer hot vapour residence times and the flow regime is completely different from that

of a fluidised bed. However, TGA is valuable as it can be used to show how samples

might behave under slow pyrolysis conditions and to estimate the relative char yields

during pyrolysis. TGA was used to determine the moisture content (physically bound

water), volatiles (including reaction water), char, ash and fixed carbon content of each

feedstock.

TGA uses only 4-5 mg of sample and large particles would not fit into the TGA

crucibles so fifteen feedstocks of less than 0.25mm were analysed. Feedstock

composition is reported to vary with different fractions of the crop [18]. Therefore,

homogenous samples were tested in duplicate to check reproducibility and averages of

reproducible results (+/- 1 wt.%) were used. TGA was carried out in two stages: Char

was produced in stage 1 using a nitrogen flow. Total combustion of the remaining char

was carried out in stage 2 to allow for the alternative determination of the ash content.

Stage 1: TGA Pyrolysis was used with nitrogen to produce char up to a maximum

temperature of 500°C. 4-5 mg of sample was pyrolysed with a heating rate of 5

°C min-1 and a hold time of 5 minutes.

Stage 2: Slow combustion of char (from stage 1) to produce ash. TGA

Combustion with air up to a maximum temperature of 575 °C, a heating rate of

2.5 °C min-1 and a hold time of 5 minutes.

Yields of volatiles and char from fast pyrolysis depend on the pr

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